Systemic lupus erythematosus (SLE) is an idiopathic autoimmune syndrome characterized by disorders of the cellular and humoral immune response that lead to autoantibody production. We have recently demonstrated that lupus lymphocytes display disease-specific, antigen receptor- initiated signaling aberrations and hypothesized that these abnormalities result in altered expression of gene(s) that impair T and B cell immune effector function. The proposed experiments, grouped in 4 specific aims, are based on the finding that lupus T cells display deficient expression of T cell receptor (TCR) zetu chain: 1. We shall test the hypothesis that TCR zetu chain deficiency is disease specific and independent of lupus disease activity. To test this hypothesis we will (a) establish the defective expression of zetu chain in lupus patients and determine its possible association with disease activity, (b) establish whether the defect is limited to lupus, and (c) establish the heritability of the disorder. 2. We shall test the hypothesis that zetu chain is exclusively defective in lupus T cells and that this defect is associated with abnormal phosphorylation of certain cytosolic proteins that are important in T cell signaling. To test this hypothesis we shall determine (a) whether other chains of the CD3 complex and the zetu protein family are deficient in lupus T cells, and (b) which proteins are involved in the increased protein tyrosine phosphorylation that is observed following crosslinking of the CD3 molecule. 3. We shall test the hypothesis that zetu chain deficiency is causally associated with the CD3-initiated hyperphosphorylation of cytosolic proteins in lupus T cells and the increased Ca2+ response. This hypothesis will be tested by (a) establishing an association between the effective expression of the zetu chain and the abnormal CD3- initiated signaling, (b) by transfecting lupus T cells with the zetu chain gene and testing whether successful transfection will reverse the described abnormal cell signaling events, and (c) determining whether the marginally CD3-mediated inositol 1,4,5 trisphosphate (IP3) production increase is due the aberrant phospholipase Cgamma and IP3 receptor phosphorylation. Finally, in the 4th specific aim we will test the hypothesis that zetu chain deficiency is the result of deletion(s) or mutation(s) of the coding or the promoter region of the gene. Experiments will be performed to consider the alternative hypothesis, i.e., zetu chain deficiency in lupus cells is the result of cell activation or other cellular regulatory abnormality.